The goal of this project is to study three aspects of the processes by which porphyrins may be effective both in location and regressions of tumors: (1)\the aggregation processes of porphyrins in aqueous solutions; (2)\the binding and incorporation of the porphyrins from the aqueous phase into membranes using liposomes as models; and (3)\the possible use of liposomes and of serum proteins (or their interference) as porphyrin delivery systems. The study is aimed at the molecular level. (1)\The dimerization equilibrium of HP, HPD, DP, MP, and PP was studied. The magnitudes obtained range from 105-107M-1. The dimerization is enthalpy-driven, delta-G~ = -8.7 Kcal/mol, delta-H~ = -11.0 Kcal/mol, delta-S~ = -7.7 e.u., for DP. (2)\Experimental procedures to study porphyrin-liposome-binding equilibrium were developed. The issues probed: (a)\binding of porphyrin monomers to PC liposomes; (b)\relationship between membrane-binding and aqueous aggregation; (c)\the aggregation state of membrane-bound porphyrin at equilibrium; and (d)\the effects of cholesterol as a membrane component. HP and HPD bind to PC liposomes with constants of 1600 M-1 and 4000 M-1, respectively (short incubation). Binding constants of HPD to cholesterol-containing liposomes were found to depend on the level of cholesterol, from 9000 to 3000 M-1 for high affinity and from 1000 to 100 M-1 for the low affinity components of HPD, with the increase in cholesterol (long incubation). The membrane-bound porphyrin was monomeric for all systems regardless of the initial aggregation state in the aqueous phase. (3)\Experimental procedures for studying the binding of porphyrins to serum albumins with consideration of the porphyrin aggregation equilibrium (using DP, BAS, and HSA) and for the competition of HSA and liposomes on HPD were developed. The binding constant of monomeric DP to HSA was determined to be 4 x 107M-1. HSA (5%) can remove HPD prebound to liposomes; the HSA-bound HPD may be accompanied by lipid molecules and/or small liposomes. (1)